1. Field of the Invention
This invention relates to a method of producing a high-strength cold-rolled steel sheet excelling in deep drawability and ductility and suitable for use in automobiles, etc.
2. Description of the Related Art
As a result of the desire to increase the quality of automobiles, the cold-rolled steel sheets used in automobiles are now required to have certain properties not previously required.
For example, there is a strong tendency to reduce the weight of the car body to attain a reduction in fuel consumption and, at the same time, to use a stronger steel sheet having a tensile strength of, for example, 35.about.65 kgf/mm.sup.2, in order to ensure the requisite safety for the occupants of the automobile.
A cold-rolled steel sheet to be used as a panel, etc. in an automobile must have an excellent deep drawability. To improve the deep drawability of a steel sheet, it is necessary for the mechanical properties of the steel sheet to be such as to exhibit a high r-value (Lankford value) and high ductility (El).
The assembly of a car body has been conventionally performed by joining together a large number of pressed-worked parts by spot welding. In recent years, there has been an increasing demand to enlarge some of these parts or convert them into integral units so as to reduce the number of separate parts and welding operations performed.
For example, the oil pan of an automobile has to be completed by welding because of its complicated configuration. But, the automobile manufactures have a strong desire to produce such a component as an integral unit. Further, to meet the increasing diversification in the needs of consumers, the design of cars has become more and more complicated, resulting in an increase in the number of parts which are difficult to form out of conventional steel sheets. To meet such demands, it is necessary to provide a cold-rolled steel sheet which is much superior to the conventional steel sheets in terms of deep drawability.
Thus, although a steel sheet for an automobile must be very strong, it is required, at the same time, to exhibit an excellent deep drawability in press working. In view of this, a study is being made with a view to developing a steel sheet which has a high level of strength and which, at the same time, exhibits a r-value equal to or higher than those of the conventional steel sheets and also, excellent ductility.
A number of methods for producing cold-rolled steel sheets satisfying the above requirements have been proposed.
Japanese Patent Laid-Open No. 64-28325 discloses a method for producing high-strength cold-rolled steel sheets according to which an ultra-low-carbon steel containing Ti-Nb and, as needed, B, is subjected to recrystallization in the ferrite region after hot rolling; then, cold rolling is performed and, further, recrystallization annealing is conducted. However, although in this method an attempt is made to attain a high level of strength through addition of Si, Mn and P, the amount of these additives is not enough. Further, because of the large amount of Ti added, a phosphide of Ti is formed in great quantities, so that the r-value obtained is rather low; and the product of the tensile strength and the r-value (TS.times.r) is 102 or less, which indicates an insufficient level of deep drawability.
Japanese Patent Laid-Open No. 2-47222 discloses a method of producing high-strength cold-rolled steel sheets according to which an ultra-low-carbon Ti-containing steel containing some B, as needed, is subjected to hot rolling in the ferrite region and then to recrystallization; after that, it is subjected to cold rolling, and then to recrystallization annealing. Although this method enables a high r-value to be obtained, the contents of solute reinforcement elements Si, Mn and P are 0.04 wt % or less, 0.52 wt % or less, and 0.023 wt % or less, respectively. Because of these low contents of the reinforcement elements, it is impossible to obtain a high strength of 35 kgf/mm.sup.2. Nor does this prior-art technique suggest any method for producing a high-strength cold-rolled steel sheet having a tensile strength of 35 kgf/mm.sup.2 or more.
Japanese Patent Laid-Open No. 3-199312 discloses a method of producing high-strength cold-rolled steel sheets according to which an ultra-low-carbon Ti-containing steel with some B, is subjected to hot rolling and then to cold rolling; after that it is subjected to recrystallization. The problem with this method is that it uses a steel containing a large amount of Ti, which is not affected by a hot-rolled sheet recrystallization process, with the result that the r-value obtained is rather low, the product of the tensile strength and the r-value (TS.times.r) being less than 105. Thus, the method does not provide a sufficient level of deep drawability.